This invention relates in general to the balancing of rotating bodies and, more specifically, to the detection and correction of very small imbalances in large rotatable bodies such as marine propellers or very large gears.
A wide variety of machines have been developed for balancing rotating bodies such as automobile wheels. These machines hold the wheel at the end of a horizontal arbor and rotate the wheel while measuring vibration caused by the unbalanced conditions. These machines do not have the capacity to permit balancing of large, heavy parts, because of the cantilevered arbor.
Present practice for checking balance of marine propellers, large gears, etc., is to insert a heavy arbor through the propeller or the like so that the ends of the arbor can be set into spaced sets of bearings or rollers. The axis of the arbor is thus horizontal and the propeller can turn about this axis while supported by the rollers. The axis of the arbor is thus horizontal and the propeller can turn about this axis while supported by the rollers. Typical of this arrangement is that shown in U.S. Pat. No. 3,478,602 to Tatum. This arrangement prevents lateral and longitudinal movement but permits rotation about the arbor axis. If one blade of the propeller is heavy, it tends to rotate the propeller until that blade is low. Balance is then achieved by grinding metal from the heavy blade until no further imbalance is detected. Unfortunately, friction in the bearings of the rollers tends to restrain rotation, restricting the accuracy of the balancing.
Attempts have been made to mount the rotatable part on a vertical arbor mounted on a cradle which is supported by flexible plates. Typical of these is the machine described by Karpchuk in U.S. Pat. No. 3,024,361. The cradle is mounted on vertical flexible plates which tend to bend when an unbalanced part is rotated on the arbor, to indicate the degree and area of imbalance. However, these flexible plates which support the weight of the cradle and rotatable part in compression must be relatively thick and sturdy. Also, the plates must be strong enough to resist the bending moment caused by rotation of a severely out-of-balance part. Thus, these flexible plates do not provide the desired degree of sensitivity and accuracy in balancing large parts such as large gears, marine propellers or the like. Another major disadvantage of this system is the required high rotational speed to detect small imbalances. This system relies on centrifugal forces of the imbalances to induce deflection sufficient for practical measuring. Rotational speeds to detect small imbalances can become hazardous when working with massive articles.
Therefore, there is a continuing need for machines for vary accurately balancing large rotatable parts.